1. Field of the Invention
The present invention relates to a light emitting display device having an indicator that emits a light by supplying a d.c. current thereto and a drive circuit for the indicator, and more particularly to a light emitting indicator drive circuit that conducts a correcting process for reducing a difference in the luminance of emitted light of an indicator having a plurality of display elements each having a different area.
2. Description of the Related Art
FIGS. 8 and 9 show a schematic diagram of a light emitting indicator drive circuit in a conventional light emitting display device and the element shape of a conventional light emitting indicator, respectively. As shown in FIG. 8, display elements 801 to 820 are connected to each other in the form of a matrix. Also, an output of a constant-current source 821 having a switching function and allowing a current to flow out is connected to the plus electrode sides of each of the display elements 801, 806, 811 and 816. Likewise, an output of a constant-current source 822 is connected to the display elements 802, 807, 812 and 817, an output of a constant-current source 823 is connected to the display elements 803, 808, 813 and 818, an output of a constant-current source 824 is connected to the display elements 804, 809, 814 and 819, and an output of a constant-current source 825 is connected to the display elements 805, 810, 815 and 820, respectively. On the other hand, a switch 826 that allows a current to flow into the ground is connected to the minus electrode sides of the display elements 801, 802, 803, 804 and 805, respectively. Likewise, a switch 827 is connected to the display elements 806, 807, 808, 809 and 810, a switch 828 is connected to the display elements 811, 812, 813, 814 and 815, and a switch 829 is connected to the display elements 816, 817, 818, 819 and 820, respectively.
As shown in FIG. 9, in the conventional example, the areas of the display elements 801 to 820 disposed within the light emitting indicator 901 are identical with each other, and the constant current sources 821 to 825 supply the same current. For that reason, the current densities of all the display elements become equal to each other, to thereby obtain substantially the same luminance.
In order to turn on/off the respective display elements, each of the constant current sources 821 to 825 has a switching function so that the respective constant current sources can be turned on/off. On the other hand, the switches 826 to 829 are sequentially turned on one by one in a time sharing manner, and there is no case where two or more switches are turned on at the same time. In the case of turning on the display element 801, the constant current source 821 and the switch 826 are turned on. Similarly, all the display elements can be selectively turned on by the combination of the constant current sources 821 to 825 with the switches 826 to 829.
With the above structure, a very large number of light emitting elements are arranged to conduct dot matrix display, thereby being capable of executing various display.
If the dot matrix display is applied to an indicator such as an arm watch which requires a small size and a low power, the power consumption of the driver and the chip size become large, and also if no fine dot matrix display is used, fine character display cannot be made. In this case, segment display is advantageous, but it is difficult to make the areas of the respective segments identical with each other, as a result of which if the segments are driven by a constant current source having the same current value, the density of current depends on the segments to produce a difference in luminance between the respective segments.
in particular, even a difference in luminance of several percent between the adjacent segments is conspicuous. For that reason, if the luminance between the respective segments is not corrected, the display quality is remarkably degraded so that the dot matrix display cannot be applied to a fashionable wristwatch or the like.
The present invention has been made to solve the above drawbacks, and therefore an object of the present invention is to provide a light emitting indicator drive circuit which is capable of correcting a difference in luminance between the respective segments to a level where there occurs no problem from the visual viewpoint.
In order to achieve the above object, according to the present invention, there is provided a light emitting indicator drive circuit in which a current value is made variable by a gate voltage of a MOS-FET which is a constant-current source that drives display segments, and the gate voltage is controlled by using segment area information, light emitting characteristic degradation information or the like, or the current is controlled by combination of the on/off states of a plurality of FETs.
According to one aspect of the present invention, there is provided a light emitting display device, comprising a light emitting indicator having a plurality of light emitting elements different in light emitting area, a constant current source that supplies a current to the plurality of light emitting elements, respectively, and a current control circuit that controls the constant current source to supply constant currents to the respective elements corresponding to the areas of the plurality of light emitting elements. With this structure, the currents which are supplied to the respective light emitting elements can be corrected, individually, without the need to increase or reduce a generated current value using a resistor or impedance element so that the light emitting luminances of the plurality of light emitting elements become substantially equal to each other, thereby being capable of reducing a variation in the luminance between the respective display elements.
Also, according to another aspect of the present invention, there is provided a light emitting display device in which a constant current FET is used as the constant current source, and the current control circuit controls the gate voltage of the constant current FET to supply the current corresponding to the areas of the plurality of light emitting elements to the respective light emitting elements. For that reason, a variation in the luminance between the respective display elements can be reduced. By utilizing a plurality of constant current FETs of different driving capabilities, a more fine control of the current is possible.
Further, according to still another aspect of the present invention, there is provided a light emitting display device in which the area information of the plurality of light emitting elements are installed in the interior of the light emitting indicator. With this structure, the current control circuit can read the area information if necessary to correct a current which is supplied to the respective light emitting elements so that the light emitting luminances of the plurality of light emitting elements become substantially constant. In the case where a constant current FET is employed as the constant current source, the current control circuit can read the area information if necessary to correct the gate voltage of the constant current FET so that the light emitting luminances of the plurality of light emitting elements become substantially constant, to thereby control a current which is supplied to the respective light emitting elements. For that reason, a variation in the luminance between the respective display elements can be reduced with high accuracy.
Still further, according to still another aspect of the present invention, there is provided a light emitting display device comprising: a deterioration detecting circuit that detects the deterioration of a light emitting characteristic; and a signal processing circuit that calculates deterioration information outputted from the deterioration detecting circuit and the area information of a plurality of light emitting elements; wherein the current control circuit reduces the deterioration of the light emitting luminance caused by the deterioration of the light emitting indicator on the basis of the output data of the signal processing circuit to correct a current which is supplied to the respective light emitting elements so that the light emitting luminances of the plurality of light emitting elements become substantially constant. In the case where a constant current FET is employed as the constant current source, the current control circuit corrects the gate voltage of the constant current FET to control a current which is supplied to the respective light emitting elements. With the above structure in which the current which is supplied to the light emitting elements can be corrected, even if the display elements are deteriorated with time, a variation in the luminance between the respective display elements can be reduced with high accuracy.